Adobe Acrobat Pro has a proper redaction feature, but it needs a paid Acrobat Pro subscription, and many people only need to hide an account number once. There are free ways to do it, and also several popular "free" methods that quietly do nothing. This guide covers the ones that work, what each one actually does to the file, and how to prove the result.
What "real" redaction has to do
The text must be removed from the file, not covered. A rectangle painted on top is only a drawing; the characters stay in the PDF and can be copied, searched or exposed by deleting the shape. For the underlying mechanics, read why black-box PDF redaction fails. Any method below must pass the same test at the end.
Method 1: Preview on a Mac
Preview has a built-in Redaction Selection tool in its Markup toolbar. Per Apple's documentation, it selects text to remove it from view, you can change the redaction while you edit, and once you close the document the redaction becomes permanent.
- Open the PDF in Preview and show the Markup toolbar.
- Choose the Redaction Selection tool and select the text to remove.
- Save, close the document, and reopen it to check.
Two cautions. Apple notes that if the PDF has a table of contents, entries that link to redacted text are not removed. And Preview only redacts text. If your PDF is a scan, there is no text to select, so the tool has nothing to act on. Treat Preview as good for short, text-based documents and always run the test at the end. Also remember that document properties (author, software, dates) are not touched by redaction.
Method 2: a browser tool that flattens pages
Because the safest redaction is the one that leaves nothing to find, some tools render each page to an image, paint over the marked areas, and rebuild the PDF from pictures. The text layer under the boxes is not hidden; it is gone, because the new PDF never had one. Our PDF redaction tool works this way.
- Open the tool and drop in the PDF. It loads on your device, so a confidential file is not sent anywhere.
- Draw boxes over the content, or use Mark matches to find a word or number and mark every occurrence.
- Choose 150, 200 or 300 dpi and JPEG or PNG page images, then export.
- Run the test below.
The trade-offs are honest ones. The output is no longer searchable, so it suits documents you share for people to read rather than documents you need to keep editable. The file can be larger. Pages become pictures, so very small text is only as sharp as the resolution you chose. And the tool does not do OCR, so a scanned PDF works fine: it is already pixels, and you simply draw over the area.
Method 3: Acrobat Pro, if you already have it
For reference, here is what Adobe's help documents. Choose All tools > Redact a PDF, then Redact text and images, drag over the content, and select Apply. In the dialog, you can toggle sanitize and remove hidden information and then save a sanitized file. Acrobat's strength is that it edits the content so the document can stay searchable and can also clear metadata, comments, attachments and hidden layers. If you have it, use it. If you do not, Methods 1 and 2 are enough for most one-off jobs.
Methods to avoid
- A black rectangle or shape. Cosmetic only.
- Black highlight. A highlight annotation sits on top, and the text is still selectable.
- Whiteout or a white box. Same problem, plus it is obvious.
- Converting to Word, editing, and exporting back. You can leave tracked changes, comments and document properties behind, and layout may break.
- Cropping. Cropping hides part of a page but usually keeps the content outside the crop box in the file.
- Online redactors that upload your file. You are handing the unredacted document to a stranger's server before it is redacted. If the file is sensitive enough to redact, prefer a tool that works on your device. Our guide to verifying uploads explains how to check.
The 60-second test for any redacted PDF
- Search. Open the file and press
Ctrl/⌘ + F. Search for a word you redacted. - Copy. Select across the redacted area, copy, and paste into a plain-text note.
- Extract. If you have a command line,
pdftotext file.pdf -prints every piece of text that is really in the file. - Properties. Check the document properties for an author, a title or a software name that you did not mean to share, or clean them with the metadata remover.
- Look at every page. Repeated headers, footers and bookmarks are where leftovers hide.
If any step finds the hidden text, the redaction was cosmetic. Start again from the original with a different method.
Which one should you choose?
| Situation | Choice |
|---|---|
| A short text PDF, you use a Mac | Preview, then test |
| Any PDF, any device, confidential, one-off | A flattening tool that runs in your browser |
| A scanned PDF | A flattening tool (Preview cannot select scanned text) |
| A long document you must keep searchable | Acrobat Pro with sanitize |
Always keep the original
Every method here writes a new file. Keep the original in a safe place and share only the redacted copy; the original still contains everything.
Sources
Try it yourself
Use the True PDF Redaction. It runs in your browser and your files stay on your device.